A multi-output power supply typically has one transformer that provides for a main output and at least one auxiliary output. Generally, the main output is regulated by a feedback circuit that controls the volt-second product of the transformer such that the main output remains relatively constant. With this regulation method, the main output typically has a total regulation of less than 0.5%, where total regulation is the sum of the line, load, and cross regulation. While the main output is tightly regulated, the auxiliary output, without some independent regulation circuit, varies considerably. It is not uncommon for an unregulated auxiliary output to have a total regulation of 15%.
There are a variety of known regulation circuits that will tightly regulate the auxiliary output (less than 5% total regulation), such as linear regulators or DC-DC converters. Linear regulators, such as the 317 series manufactured by Motorola, Inc. and others, are relatively efficient and inexpensive for auxiliary oututs having light loads, typically less than 0.5 Amp. When the load requirements exceed 0.5 Amp, cost, efficiency, and heat dissipation of a linear regulator become a primary concern and may result in a relatively expensive, thermally, and electrically inefficient auxiliary regulation circuit. For load requirements greater than 5.0 Amps, a linear regulator is generally too inefficient and/to too expensive to use, thus a DC-DC converter, or the equivalent, is used. A DC-DC converter may be as much as 90% efficient and have a total regulation of less than 1%, however, a substantial amount of components is required to achieve this. A switch mode DC-DC converter typically requires a switching element, a transformer, a regulation or a feedback circuit, and a filter.
A need exists for an auxiliary output regulation technique that will maintain acceptable main output regulation, that will be more efficient and applicable to higher current levels than a linear regulator, and that will require less components and complexity than a DC-DC converter.